I am trying to set up a Tekkit-like server (+ some different mods). Most mods are not 1.4.2-compatible, yet, so the server runs 1.3.2. I can use MCNostalgia to downgrade the client, but is there any way to downgrade the server or download an old version?

Thanks a lot! I knew it was possible to do this, but couldn't find a way to work out the URL. I just figured it out using MVC (Minecraft Version Changer, mcvc.in), a nice GUI way to download any client or server. I'll submit it as an answer, unless your submit yours. :)
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JJ56Nov 7 '12 at 1:56

MCNostalgia (http://www.minecraftforum.net/topic/800346-tool-mcnostalgia) is the generally accepted way to downgrade your Minecraft version. It works based on binary diffs, recalculated on each update for all ~80 something versions archived -- hence it requires an unmodified .jar of the current version.

Some of the available really old versions include:

indev-2-23-10

infdev-6-18-10

Alpha 1.0.4, .5_01, .11, .14, .15, .16, .17_02, .17_04

Alpha 1.1.0, .2, .2_01

After about halfway through Alpha, pretty much every version is available.